1. Field of the Invention
The present invention relates in general to thermal control devices for integrated circuits (ICs) and in particular to a self-balance thermal control device for ICs.
2. Description of Related Art
An integrated circuit (IC) generates heat. That heat has to be removed from the IC because an IC that is too hot will not operate properly. Heat escapes from an IC through some combination of radiation, conduction and convection. An IC typically loses most of its heat by conduction into objects it contacts or by convection through the surrounding air. A metallic heat sink is attached to an IC to increase the rate at which heat is carried away from the IC by conduction. A heat sink, which normally transfers the heat it acquires from the IC to the surrounding air, typically includes fins to increase its surface area, thereby increasing the rate at which it transfers heat into the air. When fans force ambient air past the heat sink fins, they increase the rate at which the heat sink is able remove heat from the IC. The heat transfer rate can be further increased by forcing chilled air past the heat sink fins.
While it is important to keep an IC sufficiently cool, in some applications it is also important to keep an IC sufficiently warm. For example the speed with which transistors within a digital IC switch on and off is influenced by the IC's temperature. Typically the cooler the IC, the faster the transistors switch. In applications where IC transistor switching speed must be tightly controlled, it is helpful to keep the IC within a limited temperature range so that switching speed is neither too fast nor too slow. If an IC generates a constant, predictable amount of heat and if the temperature and flow rate of the air around it are constant and predictable, then a properly sized heat sink could hold the IC at a desired temperature. However the amount of heat that an IC generates can vary from time to time depending, for example, on the load the IC is driving at any given moment or on the duty cycle of its input signals. Also the temperature and flow rate of the cooling air can vary over time. Thus while a heat sink can remove heat from an IC and help keep it cool, a heat sink does not necessarily hold an IC to any particular temperature range. The more heat the IC generates, the hotter it gets.
Temperature control systems employing feedback have been used to keep an IC within a limited temperature range. Such systems sense the temperature of an IC in some manner and then either control the rate at which the IC produces heat or the rate at which heat is carried away from the IC so as to keep the sensed temperature within the desired range. IC temperature is typically sensed by a sensing device either attached to the IC, mounted near the IC, or formed within the IC itself. The sensing device produces an output voltage or current that varies with temperature. That voltage or current is used, for example, to control the amount of heat produced by a heater attached to the IC or to control the temperature or flow rate of cooling air passing over the IC.
Such feedback methods for controlling IC temperature can work quite well but they can be expensive. What is needed is an inexpensive system for controlling operating temperature range of an IC.